The present invention relates to a charger for an electronic device that charges a rechargeable battery of the electronic device, an electronic device, and a charging method.
Nowadays, there are more electronic devices, such as an electronic still camera, a mobile phone, a personal digital assistant (PDA), and a portable game machine, that charge its rechargeable battery with bus power, which is supplied from another electronic device (e.g., personal computer), which serves as a connection origin, through, for example, a USB or IEEE 1394 communication cable.
Japanese Laid-Open Patent Publication No. 2006-243863 describes technology for sending input current of bus power, which is obtained from a USB bus, to a power supply line of an HDD block. A charging circuit uses some of the input current to charge a rechargeable battery. A step-up circuit increases the output voltage of the rechargeable battery, and the output current of the rechargeable battery is added to the current of the power supply line and supplied to the HDD block. This ensures that the HDD block is supplied with sufficient drive power while the value of the current flowing to the USB remain in accordance with the standardized specification.
The USB standard allows for two different currents to be supplied, 100 mA and 500 mA.
For 100 mA, a problem occurs in that when there is not enough power, a CPU of a charger in an electronic camera cannot be activated. In such a case, communication cannot be performed with an electronic device that is connected to the charger and serves as the connection origin. Thus, information (specified voltage, specified current, etc.) necessary for the charging cannot be acquired from the connected electronic device.
An AC adapter incorporating a USB connector may be used to charge a USB-applicable electronic device. In this case, the AC adapter serves as the connection origin. Thus, the electronic device cannot perform USB communication and therefore cannot determine the specified current of the AC adapter.
In this manner, a rechargeable battery cannot be stably charged when charging information, such as the specified current of another electronic device serving as the connection origin cannot be acquired. This problem does not occur only in electronic cameras and also occurs in many types of electronic devices that charge its rechargeable battery with bus power.